The present invention relates to a method for time synchronization tracking in broadband transmission systems. More particular, the invention relates to an algorithm for post-DFT time synchronization tracking in OFDM receivers. The present invention further relates to a system for performing a time synchronization tracking method within an OFDM receiver.
Orthogonal frequency division multiplexing (OFDM) has become a popular transmission method for high-speed wireless radio transmission, due to its potential for low complexity of transmitters and receivers, paired with robustness under severe multipath conditions. A more detailed discussion on OFDM is found in S. B. Weinstein and P. M. Ebert: Data transmission by frequency-division multiplexing using the discrete Fourier transform. IEEE Trans. Communication Technology, COM-19(5):628-634, October 1971.
In OFDM, a large number of closely-spaced orthogonal subcarriers are used to carry data. Each subcarrier is modulated with a linear modulation scheme (such as quadrature amplitude modulation (QAM) or phase shift keying) at a low symbol rate.
The orthogonality of the OFDM subcarriers allows for efficient modulator and demodulator implementation using inverse discrete Fourier transformation (IDFT) on the transmitter side for conversion of the signal into the time domain, and DFT on the receiver side for conversion back into the frequency domain.
Continuous reception of OFDM signals, such as in a receiver for the European digital terrestrial video broadcasting standard (DVB-T; Digital Video Broadcasting-Terrestrial; ETSI EN 300744, V1.5.1: Digital Video Broadcasting (DVB); “Framing Structure, channel coding and modulation for digital terrestrial television”, European Standard, European Telecommunications Standards Institute, 2004) requires continuous adaptation of the receiver sample time synchronization with respect to the transmitter sample timing which is referred to as time tracking, in order to prevent interference between subsequent OFDM symbols (inter-symbol interference—ISI) as well as inter-carrier interference (ICI) within individual OFDM symbols.
To avoid inter-symbol interference (ISI) in multipath fading channels, a guard interval is inserted prior to the IDFT block. During this interval, a cyclic prefix is transmitted which consists of the end of the IDFT output copied into the guard interval. If there is no multipath propagation, the receiver can select the time synchronization within a window that is the size of the cyclic prefix.
In multipath propagation environments, a transmitted signal reaches the receiver through multiple paths each of which may introduce a different delay, magnitude and phase thereby enlarging the transition time from one symbol to the next. Identifying the useful part of an OFDM symbol that contains minimum interference from adjacent symbols (inter-symbol interference) is a time synchronization task to be performed by the receiver. This task is critical to the overall receiver performance.
Time synchronization may be classified into two main categories: acquisition and tracking. Symbol time acquisition defines the task of initially finding the correct timing. Often, the symbol time acquisition is divided into two or more steps, where in the first step, coarse time synchronization is achieved. In the following steps, the time window is refined. For those successive steps, similar or identical algorithms that are used for tracking are often applied. Tracking defines the task of continuously adjusting the time window in the course of continuous reception to keep the time window at its optimum location.
Time tracking is crucial for the overall system performance. For OFDM, various methods for time tracking have been proposed. The known methods may be grouped into data assisted and non-data assisted tracking, and pre-DFT or post-DFT time tracking. Data assisted tracking makes use of known symbols in OFDM, e.g. reference symbols, also known as pilot symbols, or preambles, whereas non-data assisted tracking makes use of the correlation properties of the signal.
In DVB-T which is aimed at continuous reception, the standard does not define any preambles. Reference symbols are included in the multiplex, the standard defining so-called scattered pilots at every 12th carrier, and a smaller number of continual pilots that are present at fixed carrier locations.
Those pilot symbols are only accessible after DFT and only after some coarse time synchronization has already been established. Therefore, most initial time synchronization algorithms for DVB-T/H use the auto-correlation properties of the OFDM symbols with its cyclic extension for coarse symbol time estimation and then rely on the pilots for fine time synchronization and tracking.
Some pre-DFT, time-domain based, time tracking techniques that make use of the auto-correlation properties have been found to require relatively long averaging times to yield adequate results. Another disadvantage is that after the signal has been acquired those types of calculations are not required elsewhere in the receiver. Additionally, the performance under heavy multipath is not satisfying. Other known approaches aim to further improve the time domain correlation based method typically used for coarse time synchronization.
Two basic approaches for post-DFT based time tracking are known both using an estimate of the channel transfer function: